This invention relates to epoxy resin compositions. In a specific embodiment, the invention relates to latent-curing epoxy resin compositions which can be pre-mixed and stored but cure rapidly under subsequently-imposed curing conditions.
Polyester resins are often fabricated into molded parts using a sheet molding compound (SMC) technique. In this process, a styrene solution of carboxyl-terminated polyester is mixed with a peroxide (or other initiator), a thickening agent such as magnesium oxide, and a filler such as calcium carbonate or clay. This liquid mixture is then mixed with chopped fiberglass between two sheets of polyethylene film, and air bubbles are removed by squeeze rolls. Over 1-2 days, the viscosity increases from an initial value near 1 Pa.s to several thousand Pa.s. The increase in viscosity is caused by reaction of the carboxyl end groups of the polyester with magnesium oxide to form polymeric magnesium carboxylates. The viscosity reaches a plateau after the magnesium oxide is consumed. The SMC then has a leathery consistency, suitable for draping into a mold. The viscosity remains relatively constant for three months or longer, which constitutes the "molding window" of the SMC. If the viscosity is too low, liquid resin will squirt out of the mold during molding. If the viscosity becomes too high, the SMC will be "boardy" and difficult to drape, and it may not have sufficient flow to fill the mold completely.
Almost all commercially-available polyester resins have values of heat distortion temperature (HDT) below 120.degree. C., while much higher HDT's can be obtained with epoxy resins. An epoxy resin processable as SMC by polyester-type techniques would therefore be highly desirable. To prepare such a material would be difficult, however, because of the different cure behavior of polyesters and epoxies. Polyester resins cure by a radical mechanism based on dissociation of an initiator to give radicals which initiate copolymerization of the styrene diluent with the maleate and fumarate groups of the polyester chain. The decomposition rate of most radical initiators is very sharply dependent on temperature. Epoxy resins cure with almost all curing agents by ionic processes which are much less temperature dependent. Hence it is much more difficult with epoxies than with polyesters to obtain a stable mixture at room temperature which cures rapidly at high temperatures.
Styrene-diluted epoxy systems cured with trimellitic anhydride (TMA) have been used to produce epoxy-based SMC for certain high-temperature applications. However, the viscosity of such systems continues to increase after reaching the desired level instead of forming a plateau. The molding window of this SMC is only about 2 days unless it is refrigerated, making it impossible to ship the SMC. Other epoxy curing agents such as aromatic amines give similarly short molding windows.
It is therefore an object of the invention to provide a latent-curing epoxy resin system suitable for use in sheet-molding applications.